Dots per Inch (DPI) is the standard measurement used for printers. It refers to the amount of dots that are placed by the printer in a one inch space. This is similar to Pixels Per Inch (PPI), which is used to measure how many pixels are placed in an inch by a monitor or screen. Aspect/Contrast Ratios are used to compare height and width of paper, monitors, etc. For example 4x1 would be 4 height for every 1 width - this could be 400x100 or 800x200.
Monitors do not have dots, but do have pixels. The closely related concept for monitors and images is pixels per inch or PPI.
Old CRT type video displays were almost universally rated in dot pitch, which refers to the spacing between the sub-pixel red, green and blue dots which made up the pixels themselves. Monitor manufacturers used the term "dot trio pitch", the measurement of the distance between the centers of adjacent groups of three dots/rectangles/squares …